Why Are In-Class Initializers Restricted to Curly Braces or the Assignment Operator?

Ambiguity Prevention in In-Class Initializers

Why is it that in-class initializers are restricted to being enclosed in curly braces or using the assignment operator (=)? Unlike method declarations, which can utilize parentheses, in-class initializers must adhere to this specific syntax.

Rationale for the Restriction

This limitation is intended to circumvent potential syntactic ambiguity. Consider the following class definition:

class BadTimes {
    struct Overloaded;
    int Overloaded;            // Strange but legal data member

    int confusing(Overloaded); // Function declaration or data member initialization?
};

If parentheses were permitted for in-class initializers, this code would lead to ambiguity. On the one hand, the line "int confusing(Overloaded);" could be interpreted as a function declaration accepting a parameter of type Overloaded and returning an int. On the other hand, it could also be read as a data member initialization of type int with the value of the Overloaded data member. (This relates to the notorious "Most Vexing Parse" issue.)

Eliminating Ambiguity with Curly Braces

By enforcing the use of curly braces, this ambiguity is eliminated:

class BadTimes {
    struct Overloaded;
    int Overloaded;            // Strange but legal data member

    int confusing{Overloaded}; // Clearly a data member initialization
};

Now it's evident that confusing is an int explicitly initialized to the value of Overloaded, as there is no scope for interpreting it as a function declaration.

This syntax restriction ensures clear and unambiguous class definitions, preventing unintended consequences arising from syntactic confusion.

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